Plant defense belowground and spatiotemporal processes in natural vegetation

Root herbivores and pathogens play an important role in driving plant abundance, species diversity, and succession in natural vegetation. Subterranean plant feeders and pathogenic microorganisms interfere with basic functions of plant roots, such as resource uptake, storage of reserves, and anchoring of plants in the soil, but concepts and theories on control of herbivores and pathogens, such as the Green World Hypothesis, have been developed and applied almost exclusively for the aboveground subsystem. Root herbivores and pathogens affect spatial and temporal patterns in natural plant communities, and whether these patterns are cyclic or irreversible depends on characteristics of the root feeders, interactions with other soil or aboveground organisms, and the rate of changes in the abiotic environment. Established plants can tolerate root herbivores and pathogens at densities that are lethal to their offspring. Dispersal by seeds or rhizomes allows new cohorts to become established before root herbivores or root pathogens colonize and develop, but it provides only temporal release of plants from their natural enemies. Permanent release from root herbivores and pathogens contributes to plant invasiveness. I propose to expand the concept of plant-soil feedback by including plant defense belowground. Many plant secondary compounds are synthesized in the roots, and these chemicals could affect root herbivores, root pathogens, and their natural enemies. Plants may exert direct and indirect defense. resistance, tolerance, or dispersal to move away from the herbivores and pathogens belowground, and I propose that acknowledging trade-offs and life history strategies will enhance our capacity to predict spatiotemporal patterns in natural vegetation. Further studies in this area will enhance our understanding of plant abundance, succession, and invasions in natural communities, as well as the evolution of plant dispersal and other defensive strategies against root herbivores and pathogens in natural communities.